Aluminum is produced today predominantly from smelting alumina in aluminum reduction cells, commonly referred to as Hall-Heroult cells after the original developers Hall (in the United States) and Heroult (in France). The Hall-Heroult cells use carbon anodes to reduce alumina electrolytically to produce aluminum which collects in a pool of aluminum metal at the base of the cell.
The carbon anodes for the Hall-Heroult cells are formed from blending coke and pitch to an appropriate composition, molding, and baking. The carbon anodes are formed and then baked in a "ring" furnace. The anodes are packed with petroleum coke and fired in the pit-type ring furnace, so-called because of the circular pattern in which the baking operation is performed.
A circular operation derives from selecting sections of a ring furnace containing anodes which have been baked and are cooled down to be removed later in a coolant zone. Just ahead of the coolant zone are sections being fired. Ahead of the firing zone are preheat sections containing green carbon anodes preheated by hot flue gas coming from the firing zone. When baking is completed in the firing zone, e.g., after about 48-60 hours, burners are moved to the preheat sections which then become the firing zone. Burners are moved (after each baking cycle) down one side of the furnace building and up the other side in the opposite direction in a circular firing pattern from which the name ring furnace derives.
In the baking process in the ring furnace, hydrocarbons are driven off in the firing zone and generally are oxidized completely.
Hydrocarbons evolving from the preheat sections, on the other hand, where temperatures range down to about 250.degree. C., are oxidized only partially.
Hydrocarbons together with coke dust from the packing in the ring furnace are drawn through cracks in the flues of the furnace and into the flue gas to form an orange-brown fume characteristic of carbon baking furnaces. Hydrocarbon vapors condense as they cool to form a fume with an average particle size of about 0.6 microns. High visibility of the fume is caused by the large number of particles in the 0.1-1.0 micron range even though the total grain loading is usually less than about 0.1 grain/SCF in a typical furnace waste gas stream.
Emission control of the particles in the size range of about 0.1-1.0 micron generally poses difficult challenges. Wet systems such as high energy venturi scrubbers and wet electrostatic precipitators require extensive water treatment subsequent to the hydrocarbon removal. Dry systems such as incinerating systems involve large volumes of gas resulting in a cost of operation which is prohibitive except when the fuel value of the pollutant in the waste gas is sufficient to justify the capital expenditure or when the total volume of gas is relatively small. In carbon baking furnaces, the combustibles concentration is very small but the total gas volume is very large making incinerators not the emission control of choice.
In addition to the hydrocarbons, other pollutants such as sulfur and fluorine come from the carbon baking furnace. These other gaseous pollutants need to be cleaned from the waste gas, too.
Because of these problems, a pollutant scrubbing process needs to be developed to overcome the drawbacks of conventionally available processes and to provide the emission control necessary for the hydrocarbon and other pollutants coming from the ring furnace for baking carbon anodes.